A comparative study of intensity pulsations in a double heterostructure Al_xGa_1−xAs laser with and without an external resonator present

Abstract

This paper describes the pulsing behaviour of Al _x Ga_1−x As double heterostructure (DH) lasers with and without an external dispersive resonator present. We have investigated only those lasers which show self-sustained oscillations under dc excitation. For the solitary diode we have found pulses possessing a width at half-power points, Δτ _p, where Δτ _p=0.9 ns and a repetition frequency,f _os, ranging from 176 to 775 MHz depending on injection currentJ. Introducing a correlation time,t _c, as a measure of the stability of the pulse height we have obtained 5 <t _c < 40 ns. The maximum value oft _c occurs in the vicinity of the crossing point off ²_os andτ ⁻²_c , whereτ _c is the effective carrier lifetime. Output pulsations are accompanied by spectral shifts of the longitudinal diode cavity modes. The frequency halfwidth, Δν _Hw, ranging from 30 to 90 GHz is closely related to the maximum pulse power,P _max. Coupling the laser diode to an external resonator, pulse widths, oscillation frequencies and correlation times have been determined as functions ofJ/J _th (J _th is the threshold injection current), provided the photon roundtrip time is nearly equal toτ _c and the equivalent reflection factorR ^* of the external resonator isR ^* ∼ 0.87. By evaluating the experimental findings pulsing under dc excitation may be described, without a detailed knowledge of the reasons leading thereto, by using the concepts of a nonstationary quality factorQ _ns and time-dependent resonator losses characterized by Δν _Hw/δτ _p. Oscillations become self-sustained forQ _ns=const., i.e. the minimum photon density with which the pulses start to grow has to be time and excitation independent, in this casef ²_os is proportional toJ/J _th as well as Δτ _p to (J/J _th ⁻¹)^−1/2.

For certain ranges of injection current a double hysteresis has been found. In the upper branch of this hysteresis the output is continuous wave, occurring in a single mode of the coupled system. All other parts of theP-J curve are characterized by output pulsations. The carrier trap model proposed by Copeland allows a qualitative description of the experiments including calculations of the electron capture cross-sectionσ ₀ and the minimum pulse width Δτ _p,min estimated asσ ₀=3.6×10⁻¹⁶cm² and Δτ _{p, min}, ∼-0.9 ns. According to our measurements the dynamic loss originating in carrier consumption/ release due to traps is responsible for pulsing behaviour and double-switching along the hysteresis curve.